Cycles or Time
Abstract
Kirk and Fred discuss whether we should use time or number of cycles or total time a product has been available for use.
Key Points
Join Kirk and Fred as they discuss a question from a listener on whether it’s better to use time or cycles as a reliability metric.
Topics include:
- As with most of our discussions, the decision to use cycles or total time depends on the type of system you are measuring. In pumps, valves, and other mechanical systems, the time between use can significantly affect wear and fatigue damage as seals and bearings lubrication may evaporate or its viscosity changes with temperature.
- Using an electro-mechanical system continuously and not having many on-off cycles results in different mechanical wear effects versus using the system once daily.
- Starting a warm car engine is much less stressful and damaging than starting an engine after it has cooled down. The same is valid with electronics assemblies, especially in power electronics.
- Many products can log the cycles and total power on time, which differentiates between cycling and frequency of processes or on-off cycles.
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Show Notes
You can now purchase the most recent recording of Kirk Gray’s Hobbs Engineering 8 (two 4 hour sessions) hour Webinar “Rapid and Robust Reliability Development – 2022 HALT & HASS Methodologies Online Seminar” from this link.
Please click on this link to access a relatively new analysis of traditional reliability prediction methods article from the US ARMY and CALCE titled “Reliability Prediction – Continued Reliance on a Misleading Approach”
For more information on the newest discovery testing methodology here is a link to the book “Next Generation HALT and HASS: Robust design of Electronics and Systems” written by Kirk Gray and John Paschkewitz.
Awesome topic and well presented by podcasters. A few comments.
– Sometimes you would have an obvious ageing variable, like valve cycling, but sadly no instrumentation to measure it. It was never installed. Then you have to default to the “next best” ageing variable. I ran into that a lot in the field. My solution to that is to try and think of what variables you want to measure when you commission the equipment and put in the measuring devices for this.
– Having lots of different ageing variable is not a bad thing. However, the question is which one is the “best” to measure reliability parameters. I tried to highlight a methodology for this in one Accendo article I wrote: https://accendoreliability.com/age-related-degradation-variables-which-is-the-dominant-one/
– Lastly, there might be that ageing variable sitting there in front of you without you seeing it. Talk to your engineering peers about what you’re trying to seek and you might be surprised one might pop up.
And …yes, we have block heaters in Canada……. 😊. Also needed because if oil to viscous due to low temperatures, that start-up crank could kill your battery. You’d definitely be left out in the cold then…